1. Field of the Invention
The present invention relates to air conditioning systems and more particularly to supplementary air conditioning systems for buildings whose primary air conditioning system is a forced air system.
2. Description of the Prior Art
At the present time multi-tenant office buildings, malls and other buildings are often constructed with central air conditioning which cools by circulation of air (forced air system). Generally the air conditioning equipment is located at a room or floor for machinery. For example, in a multi-story building, one mechanical floor may serve sixteen floors, eight above and eight below, so that a 48-floor building would have three mechanical floors. The ducts for the flow of air from the floors to, and from, the mechanical floor are arranged in the core of the building. The building core also contains other service ducts, the main water pipes and the elevators.
The temperature of the air supplied by the building to the tenants' space for cooling, and the air returned from the tenants' space to the mechanical room, is controlled by a control system. Those temperatures are set to accord with Federal, State and local law and the building owner's lease commitments to his tenants. Presently, generally the cooling air is supplied at each tenant's space at 55.degree. F. and the return air from the floor to the ceiling plenum (space above the ceiling) is set at 78.degree. F. Since the ceiling plenum contains lights and other heat-producing equipment, the air returns to the building core at about 80.degree. F. The building uses cooling towers for heat rejection and chillers and fan-coil units to cool the air. The temperature on the tenants' spaces is consequently 78.degree. in the summer, and may be even warmer in those portions of the floors exposed to direct sunlight.
However, a temperature of 78.degree. F. is too warm for some equipment and material, for example library books, and sensitive electronic equipment, such as computer terminal equipment used in broker offices. A temperature of 78.degree. F., with its accompanying high humidity, is sufficient to cause mildew on library books and documents.
Some tenants require a lower air conditioning temperature, and lower humidity, than is supplied by the building. Such tenants have installed water-cooled supplemental air conditioning units which are supplied with cooling water, called "condenser water" as the water cools the condensers of the units, by the building from the main or a separate cooling tower called a "tenant cooling tower". For example, a typical rented floor of 28,000 sq. feet (rentable) would use 25 tons of supplemental air conditioning units to reduce the temperature on that floor form 78.degree. F. to 72.degree. F. The supplemental air conditioning may be supplied by five free-standing or ceiling units each of 5 tons capacity which are cooled by condenser water supplied through pipes from the building's main cooling water pipes. The buildings charge for that water, a typical charge in New York City being a flat charge of from one or two thousand dollars per ton (12,000 BTU/Hr.) of installed supplemental air conditioner capacity, each year, regardless of actual usage. A tenant having a 25-ton supplemental air conditioning system will pay, at that rate, of two thousand dollars, 50 thousand dollars a year, a 500 thousand dollars over a ten-year lease, in addition to the electrical charges for operation of the supplemental system.
Despite those high charges, some tenants have installed water-cooled supplemental air systems in order to save their books and documents, decrease the chances of computer malfunctions and improve the comfort of their employees. Generally, air-cooled units leading to outside air cannot be installed because the glass curtain or other outside walls may not be disturbed.